Dialkylnaphthalenes are useful in a wide variety of commercial applications. Certain dialkylnaphthalenes, such as 2,6-dimethylnaphthalene (2,6-DMN), are particularly useful as intermediates in the synthesis of 2,6-dimethyldicarboxylate (2,6-NDC) and 2,6-naphthalenedicarboxylic acid (2,6-NDA). Both 2,6-NDC and 2,6-NDA can be used in the manufacture of polymers such as polyethylenenaphthalate (PEN) and various copolymers of naphthalates and other materials, such as polyethyleneterephthalate (PET).
Polymers of 2,6-NDC and 2,6-NDA or copolymers incorporating these monomers (“2,6-polymers”) are known to be useful in a wide variety of commercial applications.
Films and fibers made from 2,6-polymers exhibit strength and thermal properties which are superior to films and fibers made from other polymers such as PET. These enhanced properties have led to the use of 2,6-polymers in camera films and magnetic recording tapes as well as electrical and electronic components.
2,6-polymers also exhibit high resistance to the diffusion of gases such as carbon dioxide, water vapor and oxygen. This resistance to gas diffusion makes these polymers useful in films and containers for a wide variety of food and beverage packaging applications.
The superior physical strength of 2,6-polymers also renders these polymers useful in physically demanding applications such as cords for automobile and motorcycle tires.
Unfortunately, the commercial scale synthesis of monomers such as 2,6-NDC is a complex, multi-step process. This complex process can result in a relatively high price per pound for 2,6-NDC when compared to other monomers.
The synthesis of 2,6-NDC typically includes several steps. In a typical synthesis, orthoxylene and butadiene are reacted over an alkali metal or other catalyst to yield a 5-orthotolyl pentene (5-OTP) alkenylation product. The 5-OTP is then cyclized over an acid catalyst to yield 1,5 dimethyltetralin (1,5-DMT). The 1,5 DMT is dehydrogenated over a noble metal or other dehydrogenation catalyst to yield 1,5 dimethylnaphthalene (1,5-DMN), which is subsequently isomerized to produce 2,6-DMN.
Once 2,6-DMN has been produced, it can be oxidized to produce 2,6-NDA, which is subsequently esterified to produce 2,6-NDC. This 2,6-NDC can then be polymerized in the presence of, for example, ethylene glycol, to produce PEN useful as a polymer or copolymer in applications such as those discussed above.
The foregoing seven step process to produce PEN demands that every synthesis step be selective and produce high yields of the desired end product if NDC is to be manufactured in a commercially successful manner.
Alternative synthesis schemes are desired to improve yields or reduce the number of steps required to produce monomers such as 2,6-NDC and 2,6-NDA. One such synthesis scheme includes the process step of the selective methylation of 2-monomethyl naphthalene (2-MN) directly to 2,6-DMN. Efficient conversion of 2-MN to 2,6-DMN requires the use of highly selective, high yield catalysts to render synthesis routes using this step economically attractive.
For example, Japanese patent document JP 6329564 describes the use of a ZSM-5 type ferrisilicate catalyst, obtained by direct hydrothermal synthesis, which is useful for the selective methylation of 2-MN. In this catalyst, iron is contained in the framework of the silicate, instead of aluminum. In other words, iron replaces substantially all of the aluminum present in the traditional ZSM-5 aluminosilicate framework. This process results in a catalyst type commonly referred to as an “Fe-MFI”-type catalyst. As described in this reference, such catalysts can provide for improved selective methylation of 2-MN when at least 80 percent, and preferably 90 or more percent, of the metal in the zeolitic lattice structure is iron.
The methylation performance of such an Fe-MFI catalyst obtained by direct hydrothermal synthesis is described below by Komatsu, et al., in an article titled “Selective Formation of 2,6-Dimethylnaphthalene from 2-Methylnaphthalene on ZSM-5 and Metallosilicates with MFI Structure” published in Zeolites and Related Microporous Materials: State of the Art 1994, Studies in Surface Science and Catalysis, Vol. 84, pages 1821–1828, Elsevier Science B.V. (1994). In this article, Komatsu et al. describe the use of their Fe-MFI catalyst to obtain about a 13 percent conversion of 2-MN, a selectivity to 2,6-DMN of about 49 percent, and a ratio of 2,6-DMN to 2,7-DMN in the converted product of about 1.7 to 1. The selectivity represented by a ratio of 2,6-DMN to 2,7-DMN of about 1.7, together with the reported overall conversion of 2-MN to 2,6-DMN of just over six percent in a methylation step, reported both in the Japanese patent and the article by Komatsu et al., are believed to be too low to enable an economically viable synthesis scheme incorporating this conversion step.
A similar methylation scheme is disclosed by Shu-Bin Pu and Tomoyuki Inui in their paper titled “Synthesis of 2,6-Dimethylnaphthalene by Methylation of Methylnaphthalene On Various Medium and Large-Pore Zeolyte Catalysts,” Applied Catalysis A: General 146, pages 305–316, Elsevier Science B.V. (1996).
Pu and Inui report a 2.9 percent conversion of 2-MN, with a selectivity for 2,6-DMN of about 48 percent, and a 2,6-DMN to 2,7-DMN ratio of about 1.5 using Fe-MFI catalyst obtained by direct hydrothermal synthesis. As with Komatsu's catalyst, while the use of iron in place of aluminum in the zeolitic lattice structure shows promise in terms of selectivity for 2,6-DMN, the combination of conversion and selectivity is believed to be insufficient to be economically viable.
What is needed is an improved catalyst for the selective conversion of 2-MN to 2,6-DMN that will provide a substantially higher yield of 2,6-DMN, so that a 2,6-NDC synthesis scheme incorporating this step can be commercially viable.